In recent decades, contact lenses have become a preferential alternative to other eyesight correction methods. Due to their increased popularity, it has become mandatory that contact lenses be manufactured on a large scale in order to meet consumer demand. Further, these lenses are required to be precision manufactured with low tolerances in order to provide a suitable and effective corrective lens.
The polymerization casting of axially symmetrical articles, such as contact lenses, may be performed by using a spin casting process. Spin casting has proven to be an effective way to mass produce contact lenses. In this process, a controlled quantity of a polymerizable liquid is placed into an open mold, which is then rotated about its vertical axis at a rotational speed sufficient to produce a centrifugal force that causes a radially outward displacement of the polymerizable liquid. By maintaining a controlled rotation rate, the centrifugal force caused by the rotation will cause the polymerizable liquid to adopt a generally concave shape. Once the polymerizable liquid has attained an equilibrium shape, polymerization of the liquid can be effected by any suitable means, such as heat or exposure to actinic radiation (i.e. ultraviolet light) so as to produce a solid polymeric contact lens.
The open mold used in a spin casting process is typically characterized by an outer cylindrical wall and a mold comprising an exposed concave molding cavity. The shape of the molding cavity will typically define the shape of the front surface of the finished contact lens, and may contain such desired elements as lenticulating curves, toric curves, non-spherical curves and other such features or shapes aimed at interacting with the eye, its optical processes, or eyelids in a predetermined manner.
The shape factor of the posterior or back surface of the lens is determined predominantly by the angular speed of rotation, as well as other factors such as the surface tension of the polymerizable liquid, and the acceleration due to gravity.
During the manufacture of contact lenses, the polymer is typically polymerized in a spin tube. The spin tube must be able to both present an accurate and straight inner bore for the molds and must spin around its own vertical axis with minimal run out of polymerizable liquid and minimal vibration within the system. Inconsistencies in the production of spin-cast contact lenses may be introduced by any number of manufacturing elements. For example, variance in the run out of the spin tube may affect the final contact lens. Additionally, inaccurate mounting of the spin tub into the rotation mount may introduce inaccuracies that affect the contact lens produced. Additionally, contaminants may be inadvertently introduced or system vibrations may generate a product that lacks sufficient precision (e.g. a contact lens with undesirable imperfections or defects). Furthermore, removal of the resultant lens from its mold may introduce rips and other defects to the lens.
In order to assure constant quality of the contact lenses, provisions are in place for automatic inspection of the contact lenses using industrial image processing methods. In image processing, the lenses are tested both in the mould halves and at the vacuum grips. An image processing method of this kind is described for example in EP patent 491663. Of course, not all defective contact lenses can be detected by this type of inspection, or nominal defects are established, such as bubbles formed by water spillages, which however illustrate artifacts. Furthermore, automated inspection systems are ill equipped to identify artifacts on the lens that may not be lens defects, but rather system contamination, such as dust specs and/or fibers. By having to sort out the perfect lenses, the yield is reduced, which has a negative effect on balancing the costs.
In order to reduce the negative effect of automated inspection systems, many current systems incorporate manual inspection of the lens. However, in order to manually inspect multiple aspects of a lens, the lens is transferred to multiple instruments. The multiple transference of the lens can actually damage or deform the lens as part of the inspection process. Consequently, this traditional manual inspection system generally causes an artificially inflated rate of lens loss and waste. This increased lens loss during the inspection process is particularly troublesome in a prescription lab which is manufacturing a one-off prescription lens for a particular individual.
In other words, traditional inspection systems incorporate multiple instruments, thereby introducing the possibility of lens damage during transference between instruments. Additionally, the increase in damage to the lenses generally requires the use of additional lenses. Furthermore, traditional systems often introduce drying of the lens which potentially changes the dimensions of the very lenses you are measuring. This change in lens dimensions via the process is counterintuitive when trying to measure an object in a static state.